This invention generally relates to separation of isotopes, and is specifically concerned with a method and apparatus for separating zirconium isotopes by balanced ion migration in a counterflowing electrolyte.
The use of zirconium for forming containers or fuel rod cladding for nuclear fuels is well known in the prior art. Zirconium exists as a mixture of isotopes which include zirconium 90, zirconium 91, zirconium 92, zirconium 94 and zirconium 96. Of all these isotopes, zirconium 91 is the least desirable to use in such containers or fuel rod cladding since its relatively high thermal neutron cross section causes it to absorb thermal neutrons and thereby to impede the uranium fission reaction which is desirable in an operational fuel rod assembly. In naturally occurring zirconium, zirconium 91 constitutes only about 11% of the overall weight of the metal, the balance being constituted mostly by zirconium 90 (51.5%), zirconium 92 (17%), and zirconium 94 (17.5%). However, the thermal neutron cross section of zirconium 91 is 158 times more than zirconium 90, 6 times more than zirconium 92, 16 times more than zirconium 96 and 18 times that of zirconium 94 the 11% by weight component of zirconium 91 in naturally occurring zirconium counts for 73% of the total thermal neutron cross section of naturally occurring zirconium.
The fact that zirconium 91 accounts for almost three quarters of the entire thermal neutron cross section of naturally occurring zirconium has motivated the development of various isotopic separation techniques designed to get rid of or at least reduce the amount of zirconium 91 in naturally occurring zirconium. In one such technique, a compound of zirconium is vaporized and exposed to a pulse of light generated by a C0.sub.2 laser tuned to the vibrations of the bond between either zirconium 90 or 91 and the other constituent atoms joined to the zirconium. The tuned pulses of light causes these bonds to resonate and to break, thus liberating either zirconium 90 or zirconium 91, depending upon the frequency of the chosen frequency of the light.
While such laser-induced isotopic separation has proven to be effective for its intended purpose, it is unfortunately expensive, and capable of separating only relatively small amounts of zirconium isotopes at any given time. Hence it does not lend itself to a scaled-up, bulk-separation process that is capable of inexpensively providing the large quantities of zirconium 91-depleted zirconium needed every year for the fabrication of new fuel assemblies and fuel containers.
Other methods are known which employ electrolytic forces to separate isotopes of other elements, such as potassium. In this technique, ions of naturally occurring potassium are introduced into an electrolyte, which may be an aqueous solution of HCl. The electrolyte and dissolved zirconium ions are introduced into a column filled with an inert particulate material which provides a lengthened tortuous flow path for the zirconium ions to travel through, and an electric potential is applied across the column. The voltage of this potential attracts potassium ions and hydrogen ions toward the cathode, while simultaneously creating a counterflow of chlorine ions toward the anode. The voltage is strong enough so that sufficient electrolytic force is applied to the lighter potassium ions to cause a net migration of such ions toward the cathode, but yet is not so strong as to apply such a net migration movement of the heavier ions toward the cathode. Because potassium 41 ions are approximately 5% heavier than potassium 39 ions, they are not as mobile in the fluid medium of the electrolyte, and the electrolytic force applied to them by the cathode is insufficient to overcome the forces of kinetic agitation which causes them to move randomly about the electrolyte in Brownian fashion, and the counterflow of non-potassium negative ions flowing toward the anode gradually causes these heavier potassium ions to migrate toward the anode. Because of the balance between the flow of potassium 39 ions toward the cathode and counterflow of potassium 41 ions toward the anode, no net flow of potassium ions occurs in the electrolyte. Eventually, over a period of time, the region of the electrolyte in the vicinity of the cathode will become enriched in potassium 39, while the region of the electrolyte in the vicinity of the anode will become enriched in potassium 41.
Unfortunately, while the technique of separating isotopes by balanced ion migration has the potential of inexpensively separating bulk amounts of such isotopes, its effectiveness is highly dependent upon the relative weights of the isotopes being separated, as the differences in these weights affects the average velocity of the ions of a particular isotope in a liquid medium. Accordingly, while the approximately 5% difference in weight between potassium 39 and potassium 41 allows balanced ion migration to be practically and effectively used to separate isotopes of potassium, the only 1% difference in weight between atoms of zirconium 90 (which constitutes a little over 50% of all naturally occurring zirconium) and atoms of zirconium 91 significantly impairs the ability of prior art balanced ion migration techniques to effectively separate zirconium 91 from other isotopes of zirconium. Additionally, the effectiveness of such a balanced ion migration technique is easily impaired by the presence of any fluid mixing forces in the electrolyte which may occur from convective currents generated by heat radiated by either electrode, or fluid agitation generated by bubbles of electrolytically produced gases. Finally, such balanced ion migration techniques tend to be slow in effectively separating isotopes.
Clearly, there is a need for both a method and an apparatus for separating isotopes of zirconium which is capable of effectively separating relatively large quantities of zirconium atoms from zirconium 91 atoms. Ideally, such a technique should be fast in operation, so that commercially significant amounts of zirconium deleted in zirconium 91 can be produced for use in the claddings of fuel rods used in nuclear fuel assemblies. Finally, the apparatus used to implement the zirconium isotope separation should be simple and inexpensive in construction, and relatively easy to operate.